Nonetheless, I would like to add a few things. First of all, here are the examples that I always use in order to clarify the difference between defining relative clauses and non-defining relative clauses:

1. Defining relative clause: “The girl at the bar who is smoking a cigarette keeps looking at me.”

2. Non-defining relative clause: “The girl at the bar, who is smoking a cigarette, keeps looking at me.”

In 1 there are several girls at the bar, and therefore you have to make it clear which girl you are referring to. Hence the name “defining relative clause”, as you must define who or what you are talking about.

In 2 there is only one girl at the bar, and coincidentally she is smoking a cigarette. It is not necessary to add the information about the cigarette because your friend will know who you are talking about anyway – there is only one girl. It is simply extra information, and indeed if you remove the clause then the sentence still makes perfect sense. Hence non-defining relative clause, as you are not defining anything.

Of course, it’s very clear here in our examples, but that’s not always the case. Nonetheless, if in doubt compare the sentence you are writing with these two examples and think: what am I doing here? Am I defining something or am I just adding some extra information?

Finally, I was asked about this particular clause in class:

1. “A group of archeologists who are exploring the island have discovered….”

In this example, it is a defining relative clause. We know this because there is no comma after ‘archeologists’. However, we could just as easily say:

2. “A group of archeologists, who are exploring the island, have discovered…”

In 1 we are defining what the archeologists are doing on the island and we consider it essential information. In 2 we are more focused on the fact that a group of archeologists have discovered something and the fact they are in the process of exploring the island is just given as extra information. We could happily leave it out.

Of course, this is an example where the difference in meaning between the two possible sentences is not very clear and not very important, and therefore not very good for the purpose of highlighting when, how and why we differentiate between defining and non-defining relative clauses. Well, that’s just how it goes sometimes! Focus on the clear examples given above and nine times out of ten, you’ll be fine. And as for the tenth time? Easy, don’t worry about it 🙂